Dr. Lillian Glass, author of Toxic Men, says avoiding the topic truly is a red flag. "If you have to walk on eggshells, that is a huge signal that the person may be toxic for you," she says. "If they get defensive, it means they have issues and can't communicate freely. If you bring up marriage they can say they are or they aren't ready for it, but if they give you a defensive attitude, that is a sign of future trouble."